Add Reference to Spring AMQP Remoting in docs

Beginning with 1.2.0, Spring AMQP now supports remoting over AMQP with
a proxy factory bean an invoker service exporter.

Add documentation to the Spring Framework remoting section with a link
to the Spring AMQP documentation.

Issue: SPR-10501
Merge pull request #315 from garyrussell/SPR-10501

* SPR-10501:
  Add Reference to Spring AMQP Remoting in docs